1. Field of the Invention
The present invention relates to a computer readable storage medium including an effective light source calculation program, and an exposure method.
2. Description of the Related Art
An exposure apparatus has conventionally been used when manufacturing fine semiconductor devices such as semiconductor memories and logical circuits by using the photolithography technique. The exposure apparatus projects a pattern (circuit pattern) drawn on a mask (reticle) as an original onto a substrate such as a wafer by using a projection optical system, thereby transferring the pattern onto the substrate. Recently, the micropatterning techniques for semiconductor devices have advanced, and the exposure apparatus is required to form patterns having dimensions smaller than the exposure wavelength (the wavelength of exposure light). In the formation of fine patterns like these, the image quality is affected by the illumination condition (effective light source) used to illuminate the mask. This makes it important to set an optimum effective light source.
Accordingly, a trial and error method is generally used where the best effective light source is selected among a plurality of possible ones by evaluating each corresponding optical images (aerial images). However, this method requires time and labor. Therefore, a deterministic method of the effective light source is desirable. Generally, the determination of an effective light source requires the calculation of many optical images. For example, an effective light source is two-dimensionally divided into a plurality of elements, and these elements are regarded as point light sources. Then, an optical image attributed to each point light source is calculated, and the point light sources contributing to faithful realization of the original pattern are adapted. In this way, an effective light source can be optimized. The optimization scheme of an effective light source like this is disclosed in Japanese Patent Laid-Open Nos. 6-120119, 2002-334836, and 2004-128108. However, since this method must calculate many optical images (aerial images), it takes an enormous time to determine the effective light source.